The Bouwers monocentric system shown in FIG. 1 is a well known design that can be corrected for 3rd order spherical aberration. This system comprises a spherical mirror 20, a concentric shell or lens 22 and an aperture stop 24. The surfaces 26 and 28 of the transmissive shell 22 and the mirror surface 20 are all concentric spheres having the same common center of curvature 30. Parallel light from a distant object is focussed upon an image plane 32 between the concentric shell 22 and the mirror 20.
The Bouwers system illustrated in FIG. 1 has no field aberrations except for Petzval curvature, that is the image 22 is curved. It is good for coverage of wide fields of view with uniform performance (assuming a curved image surface and an aperture stop 24 at the system center of curvature) and can work at relatively fast f-numbers. Monochromatic performance is limited by 5th order spherical aberration, which is reduced as the concentric lens is made thicker.
Monocentric relay systems having unit magnification comprising a primary and secondary concentric spherical mirror are utilized in the prior art to print from masters onto microcircuits. The object and image planes are located on equal distant sides of the common center of curvature. Such systems are sold for example under the trademark Micralign by Perkin-Elmer Corporation, assignee of this application. It is also known to place a concentric shell between the object and/or image and the reflector in these systems, the shell or shells being located between the object and the secondary mirror aligned with the center of curvature and the secondary.